FIG. 1 shows an exploded view of a load cell and base assembly taught in the commonly assigned, copending patent applications by the present inventor, entitled "Adjustable Inclined Plane Downstop For A Load Cell" and "Load Cell Assembly With Adjustable Stiffness". (A detailed description of the assembly shown is provided in the above mentioned applications, which are hereby incorporated by reference; however, those skilled in the art are believed able to make and use the subject invention without reference to such detailed description).
Load cell 10 functions as a transducer to convert applied load forces into proportionate electrical signals and is of the type known as "single-point" load cells. That is, all the load forces are applied to load cell 10 at point M. Typically, an extended platform is fixed more or less rigidly to single point M on load cell 10 in the design shown, as well as in other single-point load cell designs, so that bulky objects may be weighed.
Typically, scales with single-point load cell transducers protect the load cell from damage with a number of adjustable downstops. Usually there will be a centrally located downstop positioned under the moving end of the load cell or under the platform close to the load cell to prevent excessive deflection of the load cell under axial overloads. Movable element 20 serves this function in the design shown in FIG. 1. (By axial loads herein is meant loads normal to the plane of the platform which pass through the load point of the load cell and cause the load cell to deflect without twisting; in contrast to "torsion loads which tend to twist the load cell").
Such central downstops will protect the load cell from overloads placed in the center of the platform but are not effective against overloads applied away from the center and particularly along the periphery of the platform. Loads applied away from the platform center apply torsion loads to said load cell, and since excessive torsion loads may damage a load cell as easily as excessive axial loads, scales have typically protected load cells by arranging a plurality of downstops around the periphery of the platform. Downstops 40, shown in FIG. 1 are typical of such arrangements. Such peripheral overload protection is also shown in U.S. Pat. Nos. 4,278,139 and 4,420,054, both to Caris and both for "Weighing Apparatus With Overload Protection for Off-Center Loading", filed Aug. 3, 1979 and Oct. 23, 1981, respectively. In each of these patents downstops are positioned under the corners of a rectangular scale platform. Such downstops are disadvantageous in that each one requires a separate critical adjustment to close tolerances and in that they rely for effectiveness on a degree of flexibility in the platform and torsional flexibility in the load cell. Such flexibility is, however, incompatible with the high degree of rigidity required in these elements to achieve a high natural frequency, preferably greater than 30 Hz., and rapid response time.
Other references which teach various techniques for protecting a scale from overloads are: U.S. Pat. No. 4,058,179, to Price for "Load Cell Scale", filed June 16, 1976; which shows a scale with a platform mounted on a structure including a preloaded spring so that the platform would breakaway under overloads to protect the load cell, and U.S. Pat. No. 4,319,650, to Brendel, for "Load Cell Scale", filed Sep. 5, 1980; which shows a scale having a cantilevered platform which pivoted against preloaded springs under overload conditions so that the outboard edge of the platform was restrained by the base of the scale to prevent excessive deflection.
Thus, it is an object of the subject invention to provide a scale having a single point load cell transducer which is protected against torsion loads.
It is another object of the subject invention to provide such a scale wherein the difficulty of adjusting downstops to obtain such protection is substantially reduced or eliminated.
It is still another object of the subject invention to provide such a scale having a platform which is essentially rigidly mounted to the load cell transducer under normal load conditions.